Automatic dryer control



Oct. 18, 1966 T. R. HANSSEN AUTOMATIC DRYER CONTROL Filed Feb. 26, 1962 United States Patent C) 3,279,088 AUTOMATIC DRYER CONTROL Tore B. Hanssen, Ann Arbor, Mich., assignor to King- Seeley Thermos Co., Ann Arbor, Mich., a corporation of Michigan Filed Feb. 26, 1962, Ser. No. 175,703 9 Claims. (Cl. 34-45) This invention relates to temperature control systems and more particularly to an improved control system for an automatic dryer of the type where material is tumbled in a rotating drum or basket through which heated air is circulated.

Automatic dryers heretofore have been controlled by the trial and error selection of a predetermined time during which a heating unit such as a gas burner or an electrical heating unit is energized to produce a desired degree of dryness in the dryer load. With such control the time factor in drying lightweight loads is unusually critical since the rate of drying is relatively rapid resulting in overdrying unless great care is taken in selecting the time period in which the drying occurs. Furthermore, heavy loads are often damp following a trial and error selection of the time needed to dry. Consequently, automatic dryer controls often include means to assure overdrying of the load. A common control arrangement continues the drying of the load until the exhaust temperature of the heated air circulating through the load reaches a level which indicates that all moisture has been removed from the dryer load. Such systems, obviously are incapable of automatically controlling dryer operation to produce different degrees of dampness in the dryer load. Such control is desirable since many present day fabrics are wrinkle-free if a predetermined degree of moisture is retained or in some instances are easier to iron in a damp condition.

The present invention is directed to an improved dryer control system which automatically controls the heat input to a dryer in order to attain desired varying degrees of dampness or complete drying of the dryer load. The

improved control system reduces drying periods for a given size load and a desired dampness and is, therefore, more economical to operate. The improved system also eliminates the trial and error procedure heretofore encountered in manually adjusted temperature-time controls by replacing such controls with a single selector dial having a wide range of settings to give desired types of drying for varying sized loads including different types of fabrics. Once the dial is set for a desired degree of dryness the desired result will automatically occur, substantially regardless of the size of the load or the materials included in it. An operator can select, for example, substantially independently of the size of the load, the exact dampness desired for ironing or retain a desired dampness in wash and wear fabrics for wrinkle-free drying.

The primary objects of this invention are to improve automatic dryer control by conjointly sensing the inlet and outlet duct conditions to control the drying action; to obtain varying degrees of dampness or complete dryness in loads of variable weight and character by conjointly sensing the inlet and outlet duct temperatures, heating the dryer load until a combined temperature effect occurs in the inlet and outlet ducts, cooling the dryer load for a predetermined period and then reheating the dryer load until the combined temperature effect reoccurs in the ducts to initiate another cooling period, and repeating the control cycle until the desired degree of dampness occurs; to provide such control by including a first senser in the inlet duct of the dryer and a second senser in the outlet duct of the dryer to conjointly control responder means which start and stop a timer-motor for driving cam selector means which open and close switch immediately upstream of the inlet to the blower.

3,279,088 Patented Oct. 18 1966 means to establish cyclical heating and cooling of the dryer load until the desired degree of dryness is attained; and to provide an improved control system for obtaining such a cyclical action in which means are provided to compensate for ambient temperature and to compensate for varying voltages in the energy source of the improved control system.

With the above as well as other and in certain cases more detailed objects in view, a preferred but illustrative embodiment of the invention is shown in the accompanying drawing, throughout the several views of which corresponding reference characters are used to designate corresponding parts and in which:

FIGURE 1 is a somewhat diagrammatic view in vertical section of one form of a dryer having the dual sensers of the present invention mounted therein; and

FIGURE 2 is a schematic diagram of the control system of the present invention.

Referring now to the drawing, in FIGS. 1 and 2, a domestic dryer assembly 10 of conventional form is shown as comprising a housing 12 in which a drum or basket 14 is rotatably mounted and arranged to be driven by a motor 15, which also drives a blower 17, which forces a current of air past a heater 16 mounted in an opening 18 in the housing 12 and through apertures 20 in .the drum 14. Blower 17 may be conventionally mounted in housing 12, as will be understood. Suitable projections 21 on the inner wall of the drum 14 tumble the dryer load during rotation of the drum 14 to obtain uniform drying therein. The heater 16 of the illustrated embodiment is an electrical heating coil which by way of further example could be a gas burner unit located upstream of the rotatable drum 14.

The air passes through an inlet duct 22 in which a thermally-responsive senser 24 is located, for example, immediately upstream of the heater 16. The air is heated by the heater 16 and then passes through a load of clothes or the like carried by the rotatable drum 14 into an exhaust duct 23 and through an inlet 26 to the blower 17. The temperature of the air in the exhaust duct 23 is sensed by a thermally-responsive senser 28 located, for example, The spaced sensers 24, 28 conjointly act to control the degree of dryness desired in the dryer load in a manner to be discussed. It is assumed in the illustrated arrangement that the dual sensers 24, 28 are constructed of metallic wire having a high positive temperature coefficient of resistance although, as discussed below, with appropriate changes in the circuitry sensers having high negative temperature coefficients of resistance could be employed. It has been found, in accordance with certain of the concepts of the invention, that the use of such conjointly acting dual sensers has freed the drying action from the variables otherwise caused by load size and the material included in it.

In the illustrated embodiment of the invention, the drying action iscontrolled by the dual sensers 24, 28 through a control system comprising a pulser 32 directly controlled by the dual sensers 24, 28 and a responder 34 directly controlled by the pulser 32 and which controls the cycling of the system and so controls the supply of heat to the dryer. The construction and mode of operation of pulser 32 and responder 34 are discussed in more detail below.

In accordance with the illustrated embodiment of the present invention, a single selector dial 30 is manually adjustable to select the degree of dryness desired in the dryer load. In use, dial 30 is rotated in a counterclockwise direction from its normal illustrated off position through a range of settings, for example, from 1 to 9 cycles of operation. In one working embodiment, a setting of 8 or 9 cycles provided a substantially complete drying of all loads; a setting of 5, 6 or 7 cycles left from to 18% of moisture at all loads; a setting of 3 or 4 cycles corresponded to a moisture content of 18% to 35% at all loads; and a setting at 1 or 2 cycles left 35% to 50% of moisture at all loads. It should, of course, be appreciated that these figures are merely representative of one working embodiment of an arrangement embodying the concepts of the present invention.

The source which supplies the electrical energy for the control system and the electric heater 16 may be, for example, a usual source of alternating voltage such as is found in the home and'is representatively illustrated as a 3-wire source with a nominal 115-volt alternating potential existing between wires 36 and 38 (the neutral wire) and with a nominal 23 O-volt alternating potential existing between wires 36 and 40. A usual door switch 42 operated by the door (not shown) leading to basket 14, has pairs of contacts 44 and 46 in wires 38, 40, respectively. When the contacts 44, 46 are closed (the position occupied thereby when the door is closed) current is supplied to the primary winding.48 of a step-down transformer 50, and to blower motor 15, from the wire 36, through a normally open control switch 51, conductor 60 in parallel through winding 48 and motor 15,. and thence through contacts 44 to the neutral wire 38. Switch 51 has a spring contact blade 52 carrying a first passive contact 54 which isadapted to be engaged with a movable contact 56 carried by a spring contact blade 58 in spaced parallelism with and biased away from the spring contact .blade 52.

The main electrical heater. 16 is connected across the Wire 40 and conductor 60 through additional normally open door switch contacts 46 and through anormally open main heater switch 66 having a spring contact blade 70 carryinga contact 72 which is biased toward a contact 74 carried on a spring contact blade 76 which is in turn biased toward the contact blade 70.

Closing of the dryer door which normally but does not necessarily precede setting of dial 30, thus normally conditions the system for operatiombut does not initiate a drying operation due to'the open condition of the main control switch contacts 54, 45 and the power contacts 72, 74.

To initiate a drying action, after closure of the door switch contacts 44, 46, the selector dial 30 .is turned counterclockwise to a predetermined setting corresponding to the degree of dryness desired. The counterclockwise turning of the selector dial 30 rotates a shaft 78 attached thereto which carries a selector cam 80 having a peripheral edge portion engaging a follower arm 82 on the spring contact blade 52 portion of the control circuit switch 51. In the illustrated or off position of dial 30, follower 82 engages in notch 84 of cam 80 and control contacts 54, 56 are separated; in all other positions of dial 30, follower 82 rides upon the periphery of cam 80 and contacts 54, 56 are closed.

The shaft 78 also carries a selector cam 88 having a plurality of peripheral land portions 90 and notches 91 on itsperiphery. In the illustrated olf position,follower 92 on blade 70 of switch 66 rests upon the periphery of cam 88 and power contacts 72, 74 are separated. Rotation of the cam 88, for example, to the 5-cycle position, causes cam follower 92 to move into and out ofthe 1 through 4-cycle notches and to drop into the 5-cycle notch 91. When the follower arm 92 is in a notch 91 on the periphery of the cam, the spring contact blade 70, which is biased toward the spring contact blade 76, moves the-contact 72-int0 engagement with the contact 74 on the blade 76. The contact carrying end of the spring blade 70 continues to move downwardly after the contacts 72, 74 are in engagement to move a pusher portion 93 on the end of, but insulated at 94 from, the blade 76 out of supporting engagement with spring contact blade 95, enabling it to move downwardly under its own bias and separate contacts 95a. The contacts 95a are closed when the follower arm 92 is in engagement with a land portion on the periphery of the selector cam 88, and are separated at all other times.

Thus, with the dryer door closed, turning of dial 30 to one of its set positions, closes control contacts 54,

56 and also closes power contacts 72, 74. Closure of control contacts 54, 56 completes the previously traced circuits for the primary winding 48 of control transformer 50 and for the drum and blower motor 15, thereby putting the drum into rotation and inducing a flow of air through the inlet duct 22, and through drum 14 and thence through the outlet duct 23. This current of air, of course,

passes over and governs the temperatures of sensers 24 and 28. Energization of the step-down transformer 50 to Leonard Boddy. More particularly, pulser 32 includes.

a thermally-responsive polymetallic element 102 of generally U-shaped form having a first leg portion 104 and a second leg portion 106 in spaced parallelism lying in a common plane. They are interconnected by a cross arm 108 having an upstanding flange 110 which is provided to increase the stiffness of the cross arm to the point that.

the cross arm will not deflect to any significant degree in response to the mechanical or thermal forces to which the unit is subjected. It will be appreciated that the U-shaped form of the polymetallic element 102 renders it substantially insensitive to changes in ambient temperature. The outer end of the second leg portion 106 is secured to a fixed support as indicated by the earthing symbol 112 which does not connote electrical grounding. The outer end of the first leg portion 104 carries a movable contact 114 for cooperation with a fixed contact 116 opposite to and spaced therefrom.

The contacts 114,116 are initially closed when the control circuit is first energized and are opened by differential heating of the leg portions 104, 106 which is controlled by a voltage amplifying winding 118 and an operating winding 120 in intimate heat transfer relationship with the leg portions 106, 104, respectively, and more particularly being wound thereon. One end of the operating winding 120 connects to the leg portion 104 and contact 114 and its opposite end connects through the inlet and exhaust sensers 24 and 28 to ground. Contact 116 is directly connected to secondary winding 62 of transformer 50 through conductors 100 and 64. Consequent- 1y, energization of transformer 50 by operation of dial 30, as aforesaid, initiates a flow of current through sensers 24, 28 and winding 120. (A similar flow is also initiated through winding 118 but this may be neglected for the" time being and is later described.)

When the inlet and exhaust duct temperatures are: low the heating output of the operating winding 120 is at 1 a maximum because the sensers 24, 28 are cool and their: resistance is relatively low causing a relatively high current flow through the operating winding 120. Heat out:

put from the operating winding 120 raises the tempera-; ture of the movable contact carrying leg portion 104 relative to the leg portion 106 and ultimately causes the fixed and movable contacts 114, 116 to separate and interrupt the flow of current through the operating winding 120. The temperature differential between the movable contact carrying leg portion 104 and the leg portion 106 is then reduced and the contacts114, 116 reclose. Then the operating winding 120 again heats the 1 leg portion 104 and opens the contacts 114, 116, which thereafter reclose as before.

Thus the pulser 32 functions to maintain the contacts 114, 116 in a condition of incipient opening and closing with the temperature of the contact carrying leg portion 104 undulating about that temperature at which the contacts 114, 116 just open. More particularly, the rate of energy consumption by winding 120 may, of course, be expressed in terms of watts (E /R), and since with relatively constant radiating conditions the resistance of operating winding 120 may be assumed to be substantially constant, it follows that for any given value of the resistances of the dual sensers 24 and 28, the effective voltage appearing across the operating winding 120 is substantially constant and is insensitive to variations in the voltage output of winding 62 of transformer 50. It also necessarily follows that the effective voltage appearing between the closed contacts 114, 116 and ground remains substantially constant and insensitive to changes in source voltage.

The just-mentioned effective voltages are, however, affected by the resistance values, and hence the temperatures of the sensers 24, 28. More particularly, as the inlet and outlet temperatures rise, the resistances of the sensers 24 and 28 also increase. The increased senser resistance correspondingly reduces the magnitude of the current pulses flowing through the operating winding 120 during each closed period of contacts 114, 116. Since the wattage requirement of the operating winding 120 remains constant, this decrease in instantaneous current magnitude must be and is accompanied by an increase in the ratio between the closed and open periods of the contacts 114, 116 of the pulser 32. Thus, the net effect of the increase in the resistance of the sensers 24, 28 is to correspondingly increase the effective output voltage between the pulser contacts 114, 116 and ground.

In the illustrated arrangement, the responder 34 also comprises a U-shaped polymetallic element 121 having a pair of legs 122, 124, located in spaced parallelism in the same plane and having an interconnecting cross arm 126 which has a flange portion 128 for stiffening the polymetallic element 121 for the same reasons that the flange portion 110 was provided in the pulser unit 32. The leg portion 122 of the responder 34 carries a movable contact 130 and has the operating winding 117 wound in intimate heat transfer relationship therewith. The other leg 124 of the responder unit 34 is connected to a fixed support indicated at 134, and is also connected to ground. The contact 130 is normally open with respect to a contact 138 located relative to the U-shaped polymetallic element 121 on a cantilevered spring support arm 140 adjusted with respect to the contact 30 by a screw element 142. Winding 117 is connected to secondary winding 62 of transformer 50 through pulser contacts 114, 116, the body of pulser 32, conductor 115, and winding 117, and thence to ground through the body of responder 34. Thus, winding 117 receives current of a value determined by the output voltage of pulser 32 and, consequently, by the temperatures of sensers 24, 28. This effective current is integrated by responder 34.

At the beginning of the dryer heating action, winding 117 receives a comparatively low effective current and contacts 130, 138 remain separated. As the dryer heating action progresses, the heating effect of winding 117 progressively increases and contact 130 gradually approaches contact 138. At a critical summed value of the resistances of sensers 24, 28, governed jointly as aforesaid by temperature conditions in the inlet and output ducts of the dryer, contact 130 engages contact 138 and initiates actions which terminate the initial heating stage of the drying operation.

More particularly, closure of contacts 130, 138 completes a circuit which connects a timer-motor 150 across the secondary Winding 62 of the transformer 50. This circuit extends from winding 62, through conductors 64 and 100, motor 150, conductor 152 and thence through contacts 130, 138 and the polymetallic element 121 to 'rise and thereby cause ground. When the timer-motor 150 is energized it rotates the shaft 78 in a clockwise direction through conventional reduction gearing (not shown) to drive the selector cam 88 to terminate the initial heating stage.

Before describing the terminating action in detail, it should be noted that the control system does not require the attainment of predetermined individual temperatures in the inlet or exhaust ducts 22, 23 of the dryer assembly 10. However, when the temperatures in these two ducts the combined resistances of the thermally responsive sensers 24, 28 to reach a critical value, the effective output voltage of the pulser 32 will be sufficiently high to cause the operating winding 117 on the responder 34 to increase the temperature differential between the legs 122, 124 to cause the movable contact carrying leg 122 to deflect into engagement with the fixed contact 138. This critical combination of inlet and exhaust temperatures might occur, for example, with a 10- pound load when the inlet temperature is 300 F. and the outlet temperature F. and the corresponding dual senser resistances might be 24 and 28 ohms, respectively. By way of further example, with a Z-pound load these inlet and exhaust temperatures might be 250 F. and F., respectively, and the corresponding dual senser resistances might be 18 and 34 ohms, respectively.

It is preferred that a spread or differential exist between the effective voltage at which the contacts 130, 138 will be brought into engagement and the lower voltage at which the contacts 130, 138 will be separated. In the illustrated embodiment this is accomplished by employing a magnetic circuit including a permanent magnet 144 which has pole pieces on either side of the contact 138 and a ferrous armature 145 mounted on the contact carrying end of the arm 122 transversely to the longitudinal axis thereof. The magnetic attraction between the permanent magnet 144 and the armature 145 will produce a snap-closing action of the contacts 130, 138 as the movable contact 130 moves relative to the fixed contact 138.

More particularly, in response to an increase in the effective voltage output appearing across the operating winding 117 of the responder 34 the movable leg portion 122 heats and deflects in a direction to close the contacts 130, 138. This closing action is supplemented by the magnetic action of the permanent magnet 144 and armature 145. When the contact 130 is in a selecteddegree of proximity to the contact 138 the magnetic attraction produces a rapid movement of the contact 130 into engagement with the contact 138. When the effective voltage drops because of reduced temperatures in the inlet and outlet ducts 22, 23 in the dryer assembly, the movable leg portion 122 of the responder will cool tending to deflect in a direction to separate the movable contact 130 from the fixed contact 138. This separation is inhibited by the magnetic attraction between the permanent magnet 144 and the armature 145. However, when the effective voltage reduces a sufficient amount, the contacts 130, 138 will abruptly separate.

By virtue of this construction, an adequate increase in the combination of temperatures sensed by the thermallyresponsive sensers 24, 28 in the inlet and exhaust ducts 22, 23 produces (with a positive temperature coefficient of resistance senser) an increase in the effective output voltage from the pulser 32 of sufficient magnitude to produce engagement of the fixed and movable contacts 138, 130 of the responder unit 34. These contacts will remain closed until the sensed temperatures decrease to decrease the effective voltage output from the pulser 32 to a value such that the movable contact 130 of the responder 34 will separate from the fixed contact 138 once the magnetic circuit attraction is overcome. The effective voltage output at which the fixed and movable contacts 138, 130 are opened and closed are adjusted in the illustrated arrangement, by adjusting the position of the fixed contact 138 by deflecting the spring arm 140 with the screw element 142.

It will be appreciated that in the broader aspects of the invention various alternative arrangements may be utilized to cause the attainment of a certain combination of inlet and outlet temperatures to produce the control action attributed above to responder 34. For example, sensers 24 and 28 might be eliminated from the circuit of pulser winding 120 and introduced instead into conductor 115 in which event pulser 32 would serve only a voltage compensating function and deliver a substantially fixed effective voltage to loads connected between contacts 114, 116 and ground; and sensers 24 and 28 (modified to have negative temperature coeflicients of resistance) would serve to progressively increase the effective current fiow in winding 117 produced by the just-mentioned substantially constant effective voltage. Similarly, in its broader aspects it is within the purview of the invention to arrange the responder to cause its contacts 130, 138 to close in response to a decrease rather than an increase in the effective current supplied to winding 117 and in such an arrangement sensers having negative temperature coefficients of resistance rather than the positive coefiicient attributed above to sensers 24 and 28, could be used in the position shown for sensers 24 and 28. With this contact arrangement sensers having a positive coeflicient of resistance could be located in the alternative position mentioned above in conductor 115. Still other alternate relationships within the purview of the invention as defined in the claims hereof will occur to those skilled in the art.

Continuing with the description of the action which follows closure of responder contacts 130, 138, when timer-motor 150 is energized it turns the shaft 78 and selector cams 80, 88 in a clockwise direction toward the starting position. The initial movement of the cam 88 forces the cam follower arm 92 out of a notch 91 on the periphery onto a land portion 90 of the cam. This moves the spring contact blade 70 away from the blade 76 to open the contacts 72, 74 of the main heater switch and deenergize the main heater 16 terminating the heating action. Also, release of blade 76 enables it to deflect toward blade 95 and causes it to move and close contacts 95a. Closure of contacts 95a completes a selfholding circuit for motor 150.

Interruption of heating by the main heater 16, of

course, lowers the temperature conditions in both the inlet and exhaust ducts 22, 23 thereby reducing the resistance of the dual sensers 24, 28 below the critical summation value. This correspondingly lowers the effective voltage output of the pulser32 and reduces the effective voltage supplied to the operating winding 117 of the responder 34. After a predetermined reduction in the effective voltage occurs, as mentioned above, the movable contact carrying leg 122 of the responder deflects against the attraction of the permanent magnet to a degree to cause the fixed and movable contacts 130, 138 to snap to an open position. This opening is. without an immediate effect because, as aforesaid, the timermotor circuit holding contacts 95a remain closed to maintain the timer-motor in operation throughout the travel of the cam follower arm 92 across a given land portion 90 on the periphery of the selector cam 88. The time in which the cam follower 94 remains on the land portion 90 of the selector cam 88 may be, for example, one and one-fourth minutes. At the expiration of this time, which may be the same for all intervals between heating cycles or of successively different lengths depending upon the desired control effect, the cam follower arm 92 .drops into, in the present description, the fourth cycle notch thereby reclosing the main heater switch contacts 72, 74 and slightly thereafter the movement of the contact blade 70 will deflect the spring contact blade 76 to a degree which will move the pusher 93 away from the timermotor holding contacts 95a which will then open to deenergize the timer-motor 150.

Closing of the main heater switch 66 re-energizes the 'period, the selector cams positions, at which the follower arm 82 drops into the cam 80, thereby opening the: and deenergizing the transformer '50 dryer heater .16. When the temperatures in the .inlet and exhaust ducts 22, 23fhave risen to such an extent that the combined resistance of the thermally-responsive dual sensers 24, 28 again equals the predetermined critical value, as for example, 52 ohms, the second heating cycle is terminated in the same manner as was described with reference to the termination of the first heating; cycle.

As would be expected, the exhaust temperature will increase during the second heating cycle over the temperature obtained during the first heating cycle and, consequently, the critical summation of the resistances of the sensers 24, 28 is reached in the second cycle ata time when the inlet temperature is somewhat lower, and the outlet temperature somewhat higher, than at the end of the first heating cycle. Moreover, throughout a succession of heating cycles the aforesaid exhaust temperature progressively rises and the inlet temperature progressively falls off, each heating cycle being terminated when the. summation of the resistance value of the thermally-re.- sponsive sensers 24, 28 corresponding to the inlet and exhaust temperatures, reaches the predetermined resistance value which may be set, by way of example, at 52 ohms.

Termination of the second heating cycle restarts .the timer-motor 150 in the previously described manner and. again moves the selector earns 80, 88 in a clockwise direction toward the starting position. When the land portion on the periphery of the main heater selector cam 88 has completed its travel past the follower arm 92 the latter drops into the three-cycle notch cycle in the previously described manner. tions continue until the control system has three-cycle, two-cycle and one-cycle heating stages.

These opera-. completed the 9 2 rides upon the land portion the main heater selector cam 88. This relatively longer land portion 154 maintains the main heater. switch .66 open to provide a cooling down period, for example, three to four minutes in which air continues to circulate through the dryer assembly without being heated as the drum 14 continues to rotate. At the expiration of the cooling down 80, 88 reach their"v starting notch 84 on the selector control switch 51 and the drum and fan motor 66. This restores the system to the condition existing set to the selected singleor multi-stage position.

The previously described voltage amplifying winding 32 causes given variationsin 118 portion of the pulser the combined resistances of the dual sensers 24, 28 to produce a greater change in the effective output'voltage of the pulser 32 than would otherwise be the case and so increases the sensitivity of the system. More particularly, the winding 118, as shown, is wound in intimateheat exchange relationship with the fixed leg 106 of the pulser and isconnected to the body of the polymetallic element 102 to receive a pulse of current each time the winding receives such a pulse. The current flows from the secondary winding 62 of the step-down transformer, through the closed contacts of the pulser 32, the body 102 of the pulser and thence through the voltage; amplifying winding 118 to ground. The voltage amplifying winding 118 is weaker than the operating winding 120 wound on the movable 104 of the pulser, but its effect is to reduce the differentialin temperature between the legs 104, 106 of the pulser and by so doing to resist or delay the opening of the contacts 114, 116 of the pulser. In other words, withthe voltage amplifying winding 118, energy must be supplied. to the operating winding 120 at a higher rate (i.e.', the ratio between closed and open periods of contacts 114, 116 must be increased) in order to enable it to maintain the movable contact carrying arm 104 at such a temperature that the contacts 114, 116 are in the previously dethe effective voltage between the contacts 114, 116 and reinitiating the heating Upon completion of the last heating cycle, the follower: arm: 154 on the peripheryof at the time dial 30 wasrinitially.

operating contact carrying leg portion ground is proportional to the rate at which the operating winding 120 absorbs energy, it follows that the efiect of the voltage amplifying winding 118 is to increase the effective voltage between the contacts 114, 116 and ground and, consequently, to increase the effective voltage applied to the operating winding 117 of the responder unit 34.

Furthermore, the action of the voltage amplifying winding 118 is regenerative. That is to say, a given amount of heating of the fixed leg portion 106 by the voltage amplifying winding 118 requires a given increase in the heating action of the operating winding 120 on the movable cont-act carrying leg 104 and a given increase in the efiective voltage between the contacts 114, 116 and the ground. The last-mentioned increase acts to increase the duration of current flow through the voltage amplifying winding 118 which acts to increase the efiective heating period of the fixed leg portion 106 of the pulser requiring a still further increase in the rate at which the operating winding 120 absorbs energy, and a still further increase in the effective voltage between the contacts 114, 116 and ground. In spite of this regenerative effect, the system is stable because the operating winding 120 absorbs suiticiently greater energy than the voltage amplified winding 118 to enable it, under all conditions, to eventaully cause the contacts 114, 116 to separate and maintain an incipient opening and closing relationship.

The illustrated improved control circuit also compensates for variations in ambient conditions in the dryer assembly and variations in the heating effect of the main heater 16 of the dryer assembly caused by variations in the supply voltage. In operation, the air drawn into the dryer is, of course, subject to ambient conditions and, consequently, for example, increases in ambient temperature, tend to raise temperature conditions in both the inlet and exhaust ducts 22, 23. Unless compensated, these increases in air temperatures would cause the dual sensers 24, 28 to prematurely reach their critical summed resistance value. In the present arrangement, the pulser 32 is preferably placed in an area where it is subject to these ambient changes. Generally speaking, if the legs 104, 106 of the polymetallic element 102 are of the same length (i.e., if the pulser 32 were fully compensated for ambient conditions), it would be insensitive to ambient change. In the present arrangement, the pulser 32 is slightly under-compensated, for example, by making the fixed leg portion 106 of the polymetallic element 102 slightly shorter than the movable contact carrying leg 104. Being slightly under-compensated (for a given resistance value of the dual sensers 24, 28) the etfective voltage of the pulser 32 falls off somewhat with increases in ambient temperatures, thereby delaying the closure of the normally open responder contacts 130, 138 until the resistance of the duel sensers 24, 28 obtain values which are higher than normal by an amount sufiicient to compensate for the increase in ambient temperature. This compensation occurs since at a higher ambient temperature, the movable contact carrying leg 104 has a naturally higher temperature and, consequently, less energy needs to be supplied by the operating winding 120 in intimate heat transfer contact therewith in order to bring the movable contact carrying leg 104 of the pulser 32 to the critical temperature at which the contacts 114, 116 incipiently open and close.

Another variable which, in accordance with the present invention, is preferably compensated for, is variations in voltage of the supply for the main heater 16. These voltage variations, of course, vary the rate at which heat is supplied by the main heater 16. For example, an increase in the heating rate of the heater 16 tends to prematurely raise the inlet and exhaust duct air temperatures and this in turn tends to cause the dual sensers 24, 28 to prematurely reach their critical summed resistance value. It is desirable that an increase in the heating rate of the heater 16 should shorten the heating intervals to some extent, but it has been found desirable to minimize or somewhat reduce the degree to which the increases in the heating rate of the heater 16 tend to shorten the heating interval. This is accomplished by a voltage compensating winding 156 wound in intimate heat transfer contact with the fixed arm portion 124 of the responder unit 34. This winding is directly connected across the secondary winding 62 of the transformer 50 through a conductor 154, and the body of the polymetallic element 121 to ground. Increases in the rate at which heat is supplied to the leg 124 by the voltage compensating winding 156 tend to delay closing of the contacts 130, 138 of the responder unit 34. Therefore, though an increase in the heating rate of the main heater 16 has the net effect of shortening the heating cycles, such effect is partially and to a desired degree ottset by the voltage compensating winding 156.

While it will be apparent that the embodiment of the invention herein disclosed is well calculated to fulfill the objects of the invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. In drying apparatus, the combination of means defining a drying chamber for the material to be dried, means defining inlet and exhaust ducts communicating with said drying chamber, means for circulating air through said drying chamber and said inlet and exhaust ducts, heater means for heating the air, first resistive means for sensing the air temperature in said inlet duct, second resistive means for sensing the air temperature in said exhaust duct, thermomative switch means responsive to the combined resistance of said resistive means and effective when the temperatures jointly sensed by said first and second resistive means reaches a summed critical value to control the drying action, selector means for selecting any of a plurality of drying cycles, motor means responsive to said switch means for causing said selector means to alternately energize and de-energize said heater means during the selected drying cycle and to terminate the drying action, and voltage compensating heating means cooperating with said switch means to render actuation of said motor means directly dependent upon said critical value and insensitive to voltage changes.

2. In drying apparatus, .the combination of means defining a drying chamber for material to be dried, means defining inlet and exhaust ducts communicating with said drying chamber, means for circulating air through said chamber and said inlet and exhaust ducts, heater means for heating the air, first senser means having an electrical characteristic which varies with temperature for sensing the air temperature in said inlet duct, second senser means having such an electrical characteristic for sensing the air temperature in said exhaust duct, heater control means operable between on and off positions in response to an electrical signal, pulser means for generating signals in response to the condition of said first and second sensor means conjointly and independently of voltage variations, and responder means operatively associated with said pulser means and effective when the temperatures jointly sensed by said first and second senser means reach a summed critical value to actuate said heat control means and control the drying action.

3. In drying apparatus having means defining a drying chamber for drying material during any one of a plurality of selectable drying cycles, means defining inlet and exhaust ducts communicating with said drying chamber, means for circulating air through said drying chamber and said inlet and exhaust ducts, and heater means for heating the air, a heat control system comprising heating control means for successively energizing and de-energizing said heater means during a selected drying cycle, selection means for selecting a drying cycle, first sensor means having an electrical characteristic which varies with temperature for sensing the air temperature in the inlet duct, second sensor means having an electrical characteristic which varies with temperature first and second sensor means, the periods of de-energization being constant and the periods of energization being variable.

4. In drying apparatus having means defining a drying chamber for drying material by any one of a plurality of selectable drying cycles, means defining inlet and exmeans to alternately energize and de-energize said heater means during the selected drying cycle and to ultimately electric dryer or the form of a rotatable on-off positions, third switch means in said third circuit means operable between on and oh positions, actuating means for said itln'rd switch means including first and second sensor means connected in circuit therewith and operable to vary the ratio of on-otf time of said third switch means, one of said sensor means being located in said air inlet duct means upstream of said heating element and the other of on-otf time of said third switch means to energize said power operable setting means and selectively actuating said first switch means to the off position at the end of a drying cycle and selectively actuating said second switch means between the on and off positionsa variably selectable number of times until said first switch means reaches the off position whereby the heat supplied by said heating element is a function of both the temperature condition in the inlet duct means and the outlet duct means.

6. The invention as defined in claim 5 and wherein said first switch means comprises rotatable cam,means and movable blade means operable thereby, the periphery of said cam means having a single off position and a continuous on surface extending therearound, said second switch means comprises second rotatable cam means and movable blade means operable thereby, the periphery of said second rotatable cam means having a plurality of on positions spaced by off surfaces, and said common manually operable setting means being movable to index said second rotatable cam means to select a particular drying cycle and said common power operable setting means being thereafter movable to index said second cam means and move said second blade means from on position to on position around the periphery of said second cam means and to simultaneously index said first cam means until said first movable blade reaches the off position.

7. In a heating system for an electric dryer or the like having a drying chamber, an electric heating element, air circulating means for passing air over said electric heating element including air inlet duct means and air outlet duct means, and motor means for said air circulating means, heat,

operable to selectively open and close said first switch means, second manually and power operable switch control means for said second switch means operable ltO selectively open and close said second switch means and having a plurality of off positions and a plurality of on positions, common manually operable setting means for i said first switch means and said second switch means,

third circuit means for energizing said common power. operable setting means to move said first switch means;

and said second switch means between the on-otf positions, a third switch means in said third circuit means operable between on and off positions, and actuating means for said third switch means including first and second sensor means connected in circuit therewith and operable to vary the switch means, one of said sensor means being located in said inlet duct means and the other of said sensor means i being located in said outlet duct means.

8. A heating system for a dryer having spaced air connected in circuit with said first and second sensor:

means to produce a variable heat efl ect on said thermomotive member in accordance with the temperature conditions sensed in said dryer by said first and second 1 senser means, responder means connected in circuit with said pulser means and being operable thereby, and control means operable by said responder means to selectively control means comprising a a first circuit for energizing said motor means, first switch means in said circuit for controlling energization of said motor means, a second circuit for said electric heating: switch means, second switch ratio of on-off time of said third,

References Cited by the Examiner UNITED STATES PATENTS 4/1950 Gorsuch 34-45 3445 2,505,041 2,621,423 12/ 1952 Clark 14 Engel 3445 Morrison 34-45 Dunkehnan 34-45 Engel 34-45 Pelavin 219-20.41 Sidaris 34-45 FREDERICK L. MATTESON, JR., Primary Examiner. NORMAN YUDKOFF, WILLIAM F. ODEA,

Examiners.

D. A. TAM-BU RRO, Assistant Examiner. 

1. IN DRYING APPARATUS, THE COMBINATION OF MEANS DEFINING A DRYING CHAMBER FOR THE MATERIAL TO BE DRIED, MEANS DEFINING INLET AND EXHAUST DUCTS COMMUNICATING WITH SAID DRYING CHAMBER, MEANS FOR CIRCULATING AIR THROUGH SAID DRYING CHAMBER AND SAID INLET AND EXHAUST DUCTS, HEATER MEANS FOR HEATING THE AIR, FIRST RESISTIVE MEANS FOR SENSING THE AIR TEMPERATURE IN SAID INLET DUCT, SECOND RESISTIVE MEANS FOR SENSING THE AIR TEMPERATURE IN SAID EXHAUST DUCT, THERMOMATIVE SWITCH MEANS RESPONSIVE TO THE COMBINED RESISTANCE OF SAID RESISTIVE MEANS AND EFFECTIVE WHEN THE TEMPERATURE JOINTLY SENSED BY SAID FIRST AND SECOND RESISTIVE MEANS REACHES A SUMMED CRITICAL VALUE TO CONTROL THE DRYING ACTION, SELECTOR MEANS FOR SELECTING ANY OF A PLURALITY OF DRYING CYCLES, MOTOR MEANS RESPONSIVE TO SAID SWITCH MEANS FOR CAUSING SAID SELECTOR MEANS TO ALTERNATELY ENERGIZE AND DE-ENERGIZE SAID HEATER MEANS DURING THE SELECTED DRYING CYCLE AND TO TERMINATE THE DRYING ACTION, AND VOLTAGE COMPENSATING HEATING MEANS COOPERATING WITH SAID SWITCH MEANS TO RENDER ACTUATION OF SAID MOTOR MEANS DIRECTLY DEPENDENT UPON SAID CRITICAL VALUE AND INSENSITIVE TO VOLTAGE CHANGES. 